U.S. Pat. No. 2,946,398--Kyser et al (1976) and co-pending patent application Ser. No. 489,985, filed July 19, 1974, both assigned to the assignee of the present application, there is disclosed a method of asynchronous, volume displacement type of ink jet printing. By this method, a pressure chamber filled with ink is suddenly reduced in volume. From a nozzle connected to the pressure chamber, there is ejected a droplet of ink which then strikes a suitable recording medium. A vertical array of such pressure chamber and nozzles is preferably built to sweep across a recording medium line-by-line and upon reception of electrical signals to then discharge droplets in predetermined graphic patterns upon the recording medium.
To achieve this end, an array of pressure chambers and ink jet nozzles in the form of an ink jet print head is mounted on a carriage which moves along a raceway before a recording medium. The ink jet print head moves back and forth along the raceway and the recording medium is advanced for printing. One constraint on such a system is that the nozzles of the ink print head must be located very close to the surface of the recording medium due to the relative low speed of the ejected ink droplets. Moreover, as the ink jet print head passes across the medium, little variation in the distance between the surface of the medium and the nozzles of the ink jet print head is tolerable for acceptable print images.
Another consideration is that in any form of ink jet printing, but particularly with asynchronous ink jet printing, operation of such a system is particularly sensitive to contamination by dust or air bubbles and the drying of ink in the nozzles. One way of avoiding or reducing these difficulties is to provide for the capping and purging of the ink jet nozzles. Capping of the nozzles during non-operation prevents the drying of ink in the nozzles to maintain the proper ink viscosity for droplet ejection. (See Stone, U.S. Pat. No. 3,346,869). Entrance of impurities, such as dust and air, into the nozzles which impede the ink jet operation is avoided also. Purging allows the ink jet print head to discharge any impurities from the passages of the ink jet print head.
Furthermore, allowing the print head to intermittently eject ink droplets if any of the nozzles of the print head remain unfired for a predetermined amount of time while the print head is in operation and uncapped, avoids drying of ink in the ink jet nozzles, without substantially interfering with the printing operation.